Process for purifying nitrogen trifluoride gas

ABSTRACT

This invention relates to a process for obtaining a high purity nitrogen trifluoride gas which is used as a dry etching agent for semiconductors or a cleaning gas for CVD apparatus, etc., particularly to a process for removing oxygen difluoride. This is a process for purifying a nitrogen trifluoride gas by, after removing hydrogen fluoride from a nitrogen trifluoride crude gas, contacting with at least one aqueous solution containing one selected from the group consisting of sodium thiosulfate, hydrogen iodide and sodium sulfide.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a process for purifying a nitrogentrifluoride gas. More specifically, it relates to a process forparticularly removing oxygen difluoride from a nitrogen trifluoride gas.

(2) Description of the Prior Art

In recent years, a nitrogen trifluoride (NF₃) gas is noticed as a dryetching agent for semiconductors and as a cleaning gas for CVDapparatus, but the nitrogen trifluoride gas for use in these usepurposes is required to be pure as highly as possible.

The NF₃ gas can be prepared by a variety of methods such as theso-called molten salt electrolysis method in which NH₄ F·xHF which isprepared by acidic ammonium fluoride alone or ammonium fluoride andhydrogen fluoride as the starting materials is electrolyzed, or themethod in which ammonia and fluorine are reacted. The present inventorshave confirmed that NF₃ gases obtained by any of the methods containoxygen difluoride (OF₂) in most of the cases with a relatively largeamount. Particularly, in NF₃ gas obtained by the molten saltelectrolysis method, at most several hundred ppm or so (wherein "ppm"represents a volume standard, hereinafter the same) thereof arecontained.

It has been known that in the NF₃ gases prepared by the above methods,various compounds such as nitrogen (N₂), dinitrogen difluoride (N₂ F₂),nitrous oxide (N₂ O), carbon dioxide (CO₂) and unreacted hydrogenfluoride (HF) are contained as impurities.

Accordingly, in order to obtain a high purity NF₃ gas, it is required toremove these impurities, and various methods for purification have beenproposed. Regarding OF₂, in "Deutsche Luft und Raumfahrt",Forschungsbericht, Oktober 1966, Herstellung von Stickstoff-fluoridendurch Elektrolyse, p. 21, lines 19 to 20, there is described that OF₂ ina NF₃ gas can be reduced by using sodium sulfite and potassium iodideaqueous solutions. Also, in "Fluorine Chemistry and Industry, Advanceand Application", edited by Nobuatsu Watanabe (Published by KagakuKogyosha), p. 208 (1973), there is described the method that a NF₃ gasis washed with an aqueous sodium thiosulfate solution to remove N₂ O inthe NF₃ gas. However, there is not described that OF₂ in the NF₃ gas canbe removed.

Also, in J. Massonne, Chem. Ing. Techn. vol. 41(12), p. 695 (1969),there is described, as a method of removing N₂ F₂ in a NF₃ gas, themethod in which an aqueous solution such as of KI, HI, Na₂ S, Na₂ S₂ O₄; or Na₂ SO₃ is used to remove N₂ F₂ therefrom. However, according tothe method, for removing N₂ F₂ completely, a relatively long time isrequired so that the reaction bath becomes considerably large as well asa large amount of chemicals is required. In this literature, no removalof OF₂ is described.

The present inventors have established the undermentioned quantitativemethod of OF₂ which uses a low temperature gas chromatography andconfirmed that the content of OF₂ in a NF₃ gas sometimes reaches even toseveral hundred ppm.

Since the OF₂ is an oxygen-containing compound, it can be estimated thatit is extremely dangerous in the step of purifying a NF₃ gas.Accordingly, in the case of purifying a NF₃ gas, it is necessary topossibly remove OF₂ at a relatively initial stage.

Also, if OF₂ remains in a NF₃ gas, it is inconvenient for furthertreating the NF₃ gas to obtain a high purity NF₃ gas, Moreover, if suchan oxygen-containing compound is contained, it involves the problem thatit exerts bad effect when the NF₃ gas is used as a dry etching agent forsemiconductors or as a cleaning gas for CVD apparatus.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for purifyingnitrogen trifluoride gas in which oxygen difluoride in the nitrogentrifluoride gas can be removed with extremely good efficiency andeconomically.

That is, the present invention is a process for purifying a nitrogentrifluoride gas which comprises removing hydrogen fluoride from anitrogen trifluoride gas containing hydrogen fluoride and oxygendifluoride by 100 ppm or lower, or using a nitrogen trifluoride gascontaining 100 ppm or less of hydrogen fluoride, said nitrogentrifluoride gas being contacted with at least one aqueous solutioncontaining one selected from the group consisting of sodium thiosulfate,hydrogen iodide and sodium sulfide.

For removing hydrogen fluoride, it is preferred to control theconcentration of hydrogen fluoride to 100 ppm or lower by contacting anitrogen trifluoride gas with water or an aqueous NaOH solution.

For removing oxygen difluoride, it is preferred that the NF₃ gas iscontacted with the aforesaid at least one aqueous solution containingone selected from the group consisting of sodium thiosulfate, hydrogeniodide and sodium sulfide at a temperature of 0° to 20° C.

For the operation of the gas-liquid contact, it is preferred to use agas scrubber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is described as being a schematic vertical section of a gasscrubber apparatus for the operation of gas-liquid contact.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the present invention will be described in moredetail.

NF₃ gases can be prepared by various methods such as the so-calledmolten salt electrolysis method in which NH₄ F·xHF prepared by acidicammonium fluoride alone or ammonium fluoride and hydrogen fluoride asthe starting materials is electrolyzed, or the method in which ammoniaand fluorine are reacted.

In the present invention, HF in a NF₃ gas should firstly be removed. Themethod of removing the HF is not particularly limited, but, for example,there may be employed the method in which, as shown in FIG. 1, by usinga gas scrubber, water or an alkaline aqueous solution such as of sodiumhydroxide, etc. is circulated in the gas scrubber and the NF₃ gas iscontacted with the water or the alkaline aqueous solution so that HF isabsorbed by the water or the above aqueous solution to remove it (inthis case, the procedure is usually carried out with a concentration ofthe aqueous solution of generally 0.1 to 1% by weight) or the method inwhich by using a compound such as sodium fluoride (NaF) as an adsorbent,and the adsorbent and NF₃ gas are contacted at a temperature of 100° C.or so to adsorb and remove HF, and the like. In the step of removing HF,it is preferred that HF in a NF₃ gas is removed as much as possible. Itis required to remove it at least by 100 ppm or less. It is needless tosay that if a NF₃ gas containing 100 ppm or less of HF is used from thebeginning, no HF removing process is required.

As the temperature of contacting the NF₃ gas with water or an aqueousalkaline solution, as in the removing process of OF₂ as mentioned below,it is preferred as low as possible in such a range that no hindrance iscaused to the operation. But considering the facts that to use water ismore economical and the OF₂ removing process is present thereafter, itis preferred in the range of, for example, 15° to 30° C. or so.

The NF₃ gas freed of HF mentioned above is then contacted with at leastone aqueous solution containing one selected from the group consistingof sodium thiosulfate (Na₂ S₂ O₃), hydrogen iodide (HI) and sodiumsulfide (Na₂ S) to remove OF₂ contained therein.

The aqueous solution of Na₂ S₂ O₃, HI or Na₂ S (hereinafter simplymentioned as "the aqueous solution") to be used in the present inventioncan be easily obtained, in general, by dissolving commercially availablethese compounds for industrial use in water.

In the present invention, this aqueous solution can be used at aconcentration of from about 0.1N to a saturation solubility level. Theaqueous solution can be used at 0.1N to 2N. Preferably it is from 0.5Nto 2N. If the concentration is less than 0.5N, removing rate of OF₂becomes too late so that OF₂ cannot sufficiently be removed. To thecontrary, if the concentration of the aqueous solution is at theneighbor of the saturation solubility, it is not economically advisable.Also, in the present invention, as mentioned below, contacting the NF₃gas with the aqueous solution is carried out by using a gas scrubber, soit is not preferred since it becomes cause of trouble such as clog ofpipings of these apparatus.

The temperature of contacting the NF₃ gas with the aqueous solution isnot particularly limited, but if an amount of water in the NF₃ gas aftercontacting the aqueous solution is made as little as possible,operations in removing the other impurities become easy. Accordingly,the above contacting temperature is preferred as low as possible in sucha range that no hindrance is caused to the operation, and it is usuallycarried out at a temperature of 0° to 20° C.

In the present invention, as the method of contacting the NF₃ gas withthe aqueous solution, the method in which, by using a gas scrubber asshown in FIG. 1, the NF₃ gas is ventilated in the gas scrubber whilecirculating the above-mentioned aqueous solution in the gas scrubber, ispreferably used.

In the method of using the gas scrubber, if a bed of packing is providedin the gas scrubber, filled with a Raschig ring, a nash ring, a teralet,etc., it is preferred since contacting the NF₃ gas with the circulatingaqueous solution becomes good. Regarding the gas scrubber shown in FIG.1, it is explained in Example 1 below.

Also, in this method, there is a merit that by varying the circulatingamount or concentration of the above aqueous solution in correspondenceto change in OF₂ concentration in the NF₃ gas, OF₂ can be effectivelyremoved.

The time of contacting the aqueous solution with the NF₃ gas may varydepending on the concentration or circulating amount of the aqueoussolution and the OF₂ content in the NF₃ gas. OF₂ contained can besuitably removed with the contacting time of about 2 to 1000 seconds.

The pressure of NF₃ ventilated in the gas scrubber is not particularlyheightened, and if the pressure is capable of ventilating in the gasscrubber, a lower pressure is preferred since easily operable.

In the present invention, as mentioned above, it is preferred that HFcontained in the NF₃ gas when contacting with the aqueous solution ispreviously removed as much as possible. It is required to remove it atleast by 100 ppm or lower.

If HF is contained in the NF₃ gas, it is reacted with Na₂ S₂ O₃, etc. inthe aqueous solution to form a fluoride such as sodium fluoride, etc. sothat the concentration of Na₂ S₂ O₃, etc. in the aqueous solutionbecomes dilute. Thus, there causes the problem of inhibiting removalefficiency of OF₂. Also, there is the problem that the formed fluoridecauses clog of pipings of the gas scrubber, etc.

The NF₃ gas freed of OF₂ mentioned above is then purified by removingimpurities such as N₂, N₂ F₂, N₂ O, CO₂, etc. by conventionally knownmethods such as the method of contacting with Zeolite Molecular Sieves,etc. so that an extremely high purity NF₃ gas can be obtained.

Next, the analysis method of OF₂ in a NF₃ gas will be explained.

OF₂ in a NF₃ gas is analyzed by using a low temperature gaschromatography.

That is, in the case of using silica gel as a filler of the column ofthe low temperature gas chromatography, the temperature of the column iscooled from to -80° C. or so. Thereafter, a NF₃ gas containing OF₂ isinjected into the column of the above gas chromatography, and then thetemperature of the column is raised at a rate of 5° C./min. This is theso-called temperature rising analysis method from a low temperature. Thechromatograph obtained by the method gives peaks in the order of N₂,OF₂, NF₃, and further N₂ F₂, N₂ O and CO₂. Accordingly, the content ofOF₂ can be determined by the area percentage of the detected peak andthe peak of the standard sample.

As described in detail hereinabove, the present invention is a methodfor removing OF₂ in a NF₃ gas and is a simple method which comprisesusing a NF₃ gas containing 100 ppm or less of HF from the beginning orremoving HF from NF₃ gas containing HF and OF₂ and contacting the NF₃gas with an aqueous solution of Na₂ S₂ O₃, etc. By enforcement of thepresent invention, OF₂ in a NF₃ gas can be selectively, efficiently andeconomically removed.

The present inventors have succeeded by the method of the presentinvention in obtaining a NF₃ gas containing substantially no OF₂, andthis is an extremely epoch-making matter.

The NF₃ gas containing substantially no OF₂ is then purified byconventionally known methods such as contacting with Zeolite (MolecularSieves), etc. to remove impurities such as N₂, N₂ F₂, N₂ O and CO₂contained so that an extremely high purity NF₃ gas can be obtained.

When the thus extremely high purity NF₃ gas is used particularly as adry etching agent for semiconductors or a cleaning gas for CVDapparatus, it is clear that extremely good effect than before isexpected.

Also, in the preparation of NF₃ gas, particularly, safeness at thepurifying process is extremely improved so that it can be largelyevaluated the effect that preparation of NF₃ gas can be realized stably.

EXAMPLES

In the following, the present invention will be explained in more detailbe referring to the Examples, but it is sure that the present inventionis not limited by these Examples. In the following, % and ppm eachrepresent a volume standard otherwise specifically mentioned.

(Example 1)

A NH₄ F·HF molten salt was electrolyzed to obtain a NF₃ gas. When theresulting NF₃ gas was analyzed with a low temperature gas chromatographyto obtain the result of the OF₂ content being 740 ppm. Also, the contentof HF was 550 ppm. By using a gas scrubber shown in FIG. 1, HF containedin the NF₃ gas was removed.

In FIG. 1, 1 is a gas scrubber, 2 is water or an aqueous solution, 3 isa bed of packing, 4 is a NF₃ gas inlet tube, 5 is a NF₃ gas outlet tube,6, 9, 10 and 12 are pipings, 7 and 11 are pumps, 8 is a storage tank, 13is a sprinkle tube, 14 is an exhaust tube, and 15 is a supplementaltube.

That is, water was filled in the storage tank 8, and the water wassprinkled from the sprinkle tube 13 through the piping 12 by the pump 11with a flow amount of 100 ml/min. Also, water 2 in the gas scrubber 1was exhausted to out of the system through the piping 9 and the exhausttube 14 by the pump 7. Further, to the storage tank 8, water wassupplemented from the supplemental tube 15 with the amount equivalent tothe exhausted one to maintain the water level in the gas scrubber 1constantly. The gas scrubber 1 had a height of 2 m and an inner diameterof 50 cm, and in the bed of packing 3, Raschig ring (both of diameterand height of 8 mm) was filled with a height of 500 mm.

At this state, the NF₃ gas containing HF obtained by the above methodwas ventilated from the NF₃ gas inlet tube 4 into the gas scrubber 1with a flow amount of 500 Nml/min and taken out from the NF₃ gas outlettube 5 to remove HF in the NF₃ gas.

The HF content in the NF₃ gas taken out from the gas scrubber 1 was, asshown in Table 1, 42 ppm.

Next, by using the same gas scrubber as mentioned above, 10 liters of a1N Na₂ S₂ O₃ aqueous solution was charged into the storage tank 8 underthe conditions as shown in Table 1. In the same manner as the aboveremoval of HF, that is, by using the pump 11, the Na₂ S₂ O₃ aqueoussolution was sprinkled to the gas scrubber 1 with a flow amount of 100ml/min. Also, the aqueous solution 2 in the gas scrubber 1 wascirculated to the storage tank 8 through the piping 9 by the pump 7 tomaintain the water level in the gas scrubber 1 constantly. And at thisstate, the NF₃ gas freed of HF was ventilated continuously from the NF₃gas inlet tube 4 into the gas scrubber 1 and taken out from the NF₃ gasoutlet tube 5.

After 5 hours of the ventilation, the gas was low temperature collectedfrom the NF₃ gas outlet tube 5, and analyzed by a gas chromatography. Asa result, the content of OF₂ was, as shown in Table 1, 10 ppm or less.

Also, Na₂ S₂ O₃ was supplemented to the storage tank 8 once per day sothat the concentration of Na₂ S₂ O₃ in the storage tank 8 was maintainedat 1N, and removal of OF₂ was carried out continuously for 240 hours.But the content of OF₂ in the NF₃ gas ventilated was maintained at 10ppm or less and no trouble was caused.

(Examples 2 and 3)

In place of the Na₂ S₂ O₃ aqueous solution, aqueous solutions with kindsand concentrations shown in Table 1 were employed. Also, by using NF₃gases having the compositions as shown in Table 1, removals of HF andOF₂ in each NF₃ gas were carried out in the same manner as in Example 1under the conditions shown in Table 1.

The content of OF₂ in each NF₃ gas ventilated in the gas scrubber 1 atafter 5 hours of the ventilation at the OF₂ removal process after the HFremoval process was, as shown in Table 1, 10 ppm or less, respectively,and no trouble was caused.

                                      TABLE 1                                     __________________________________________________________________________                                    Example 1                                                                           Example 2                                                                           Example 3                         __________________________________________________________________________    Aqueous solution                                                                        Kinds                 Na.sub.2 S.sub.2 O.sub.3                                                            HI    Na.sub.2 S                                  Concentration (Normal)                                                                              1.0   0.5   0.5                               Conditions of                                                                           Liquid amount of storage tank 8 (l)                                                                 10    10    10                                scrubber  Aqueous solution circulated amount (ml/min.)                                                        100   100   100                                         Temperature of aqueous solution (°C.)                                                        15    15    15                                Ventilation                                                                             NF.sub.3 gas flowing amount (Nml/min.)                                                              500   500   500                               conditions                                                                              NF.sub.3 gas pressure (kg/cm.sup.2 -G)                                                              0.2   0.2   0.2                               Gas  Crude        NF.sub.3 (%)  55    55    55                                analyzed                                                                           gas          N.sub.2 (%)   43    43    43                                values            HF (ppm)      550   510   540                                                 OF.sub.2 (ppm)                                                                              740   710   720                                    HF after water scrubber ventilation (ppm)                                                                42    40    45                                     Gas          HF (ppm)      10 or less                                                                          10 or less                                                                          10 or less                             after        OF.sub.2 (ppm)                                                                              10 or less                                                                          10 or less                                                                          10 or less                             venti-                                                                        lation                                                                   __________________________________________________________________________     Note                                                                           (1) Crude gas means NF.sub.3 gas as it were obtained by the molten salt      electrolysis method.                                                          (2) Gas after ventilation means NF.sub.3 gas after ventilating the gas        scrubber in which an aqueous solution is circulated.                     

(Comparative examples 1 to 3)

By using NF₃ gases having the compositions shown in Table 2, which hadbeen obtained by the molten salt electrolysis method, and withoutpreviously removing HF in each gas, removal of OF₂ in each NF₃ gas wascarried out by using the gas scrubber used in Example 1 under theconditions shown in Table 2 in the same manner as in Example 1.

NF₃ gases before ventilation of the gas scrubber 1 and after 5 hours ofthe ventilation were analyzed by the low temperature gas chromatographyand the results were shown in Table 2. From the results in Table 2, itcan be understood that the removing ratio of OF₂ becomes low when thecontent of HF in a NF₃ gas is much.

Also, in Comparative example 1, Na₂ S₂ O₃ was supplemented to thestorage tank 8 once per day so that the concentration of Na₂ S₂ O₃ wasmaintained at 1N, and removal of OF₂ was carried out continuously. As aresult, after 160 hours, the piping 9 to circulate the Na₂ S₂ O₃ aqueoussolution into the storage tank 8 was clogged. When the cause wasinvestigated, scale due to sodium fluoride was found at the inside ofthe piping 9.

This kind of scale is considered to be formed by reacting HF in the NF₃gas and Na₂ S₂ O₃.

                                      TABLE 2                                     __________________________________________________________________________                                    Comparative                                                                          Comparative                                                                          Comparative                                                     Example 1                                                                            Example 2                                                                            Example 3                       __________________________________________________________________________    Aqueous solution                                                                        Kinds                 Na.sub.2 S.sub.2 O.sub.3                                                             HI     Na.sub.2 S                                Concentration (Normal)                                                                              1.0    0.5    0.5                             Conditions of                                                                           Liquid amount of storage tank 8 (l)                                                                 10     10     10                              scrubber  Aqueous solution circulated amount (ml/min.)                                                        100    100    100                                       Temperature of aqueous solution (°C.)                                                        15     15     15                              Ventilation                                                                             NF.sub.3 gas flowing amount (Nml/min.)                                                              500    500    500                             conditions                                                                              NF.sub.3 gas pressure (kg/cm.sup.2 -G)                                                              0.2    0.2    0.2                             Gas  Crude        NF.sub.3 (%)  55     55     55                              analyzed                                                                           gas          N.sub.2 (%)   43     43     43                              values            HF (ppm)      470    520    580                                               OF.sub.2 (ppm)                                                                              670    710    700                                  Gas          HF (ppm)      30     29     41                                   after        OF.sub.2 (ppm)                                                                              40     31     48                                   venti-                                                                        lation                                                                   __________________________________________________________________________     Note                                                                          (1) Crude gas means NF.sub.3 gas as it were obtained by the molten salt       electrolysis method.                                                          (2) Gas after ventilation means NF.sub.3 gas after ventilating the gas        scrubber in which an aqueous solution is circulated.                     

What is claimed is:
 1. A process for purifying a nitrogen trifluoride gas containing at least oxygen difluoride which comprises removing hydrogen fluoride to 100 ppm or less in the nitrogen trifluoride gas only when a nitrogen trifluoride gas contains more than 100 ppm hydrogen fluoride and then said nitrogen trifluoride gas being contacted with at least one aqueous solution containing a component selected from the group consisting of sodium thiosulfate, hydrogen iodide and sodium sulfide to remove oxygen difluoride.
 2. A process for purifying a nitrogen trifluoride gas according to claim 1 wherein the concentration of hydrogen fluoride is made 100 ppm or lower by contacting the nitrogen trifluoride gas with water or a sodium hydroxide aqueous solution.
 3. A process for purifying a nitrogen trifluoride gas according to claim 1 wherein the temperature of contacting the nitrogen trifluoride gas with the aqueous solution is 0° to 20° C.
 4. A process for purifying a nitrogen trifluoride gas according to claim 1 wherein said aqueous solution is circulated through a gas scrubber provided with a bed of packing in a vessel through which said OF₂ containing NF₃ gas is ventilated to contact the nitrogen trifluoride gas with the aqueous solution.
 5. A process of removing oxygen difluoride from a nitrogen trifluoride gas containing a mixture of oxygen difluoride and hydrogen fluoride, said process comprising;(a) removing the hydrogen fluoride from the nitrogen trifluoride gas whereby the nitrogen trifluoride gas contains 100 ppm or less hydrogen fluoride, and (b) removing the oxygen difluoride by contacting said nitrogen trifluoride gas of step (a) with an aqueous solution containing at least one component selected from the group consisting of sodium thiosulfate, hydrogen iodide and sodium sulfide.
 6. The method of claim 5 wherein said hydrogen fluoride is removed by contacting said gas with water or an aqueous solution of sodium hydroxide.
 7. The method of claim 5 wherein said aqueous solution is a 0.1N to 2N solution of said component.
 8. The method of claim 5 wherein said aqueous solution is a 0.5N to 2N solution of said component.
 9. The process of claim 5 comprising introducing said nitrogen trifluoride gas into a gas scrubber and circulating said aqueous solution through said gas scrubber to contact the aqueous solution with said nitrogen trifluoride gas, wherein said gas scrubber contains a bed of packing.
 10. A process of purifying a nitrogen trifluoride gas containing oxygen difluoride or a mixture of oxygen difluoride and hydrogen fluoride, said process comprising:(a) removing hydrogen fluoride from said nitrogen trifluoride gas when said gas contains more than 100 ppm hydrogen fluoride to a level of less than 100 ppm hydrogen fluoride; and (b) contacting said nitrogen trifluoride gas containing less than 100 ppm hydrogen fluoride with an aqueous solution of at least one component selected from the group consisting of sodium thiosulfate, hydrogen iodide and sodium sulfide to remove the oxygen difluoride. 